Yeast organisms produce a number of proteins that have a function outside the cell. Such proteins are referred to as secreted proteins. These secreted proteins are expressed initially inside the cell in a precursor or a pre-form containing a pre-peptide sequence ensuring effective direction (translocation) of the expressed product across the membrane of the endoplasmic reticulum (ER). The pre-peptide, normally named a signal peptide, is generally cleaved off from the desired product during translocation. Once entered in the secretory pathway, the protein is transported to the Golgi apparatus. From the Golgi, the protein can follow different routes that lead to compartments such as the cell vacuole or the cell membrane, or it can be routed out of the cell to be secreted to the external medium (Pfeffer et al. (1987) Ann. Rev. Biochem. 56:829–852).
Insulin is a polypeptide hormone secreted by β-cells of the pancreas and consists of two polypeptide chains, A and B, which are linked by two inter-chain disulphide bridges. Furthermore, the A-chain features one intra-chain disulphide bridge.
The hormone is synthesized as a single-chain precursor proinsulin (preproinsulin) consisting of a prepeptide of 24 amino acid followed by proinsulin containing 86 amino acids in the configuration: prepeptide-B-Arg Arg-C-Lys Arg-A, in which C is a connecting peptide of 31 amino acids. Arg-Arg and Lys-Arg are cleavage sites for cleavage of the connecting peptide from the A and B chains.
Three major methods have been used for the production of human insulin in microorganisms. Two involve Escherichia coli, with either the expression of a large fusion protein in the cytoplasm (Frank et al. (1981) in Peptides: Proceedings of the 7th American Peptide Chemistry Symposium (Rich & Gross, eds.), Pierce Chemical Co., Rockford, Ill. pp 729–739), or use a signal peptide to enable secretion into the periplasmic space (Chan et al. (1981) PNAS 78:5401–5404). A third method utilizes Saccharomyces cerevisiae to secrete an insulin precursor into the medium (Thim et al. (1986) PNAS 83:6766–6770). The prior art discloses a limited number of insulin precursors which are expressed in either E. coli or Saccharomyces cerevisiae, vide U.S. Pat. No. 5,962,267, WO 95/16708, EP 0055945, EP 0163529, EP 0347845 and EP 0741188.
Circular Dichroism (CD) is used to determine protein stability and relative stabilities of molecules. CD observed below 240 nm is due to the peptide amide chromophore and may be used to estimate protein secondary structure (Johnson (1988) Ann. Rev. Biophys. Chem. 17:145–166). The spectrum of insulin is characterized by minima at 220 and 209 nm, a negative to positive crossover near 203 nm, and a maximum at 195 nm. Upon denaturation, the negative CD in the 240–218-nm range gradually diminishes, consistent with the loss of ordered secondary structure that accompanies protein unfolding. Consequently, the folding stability of an insulin precursor may be quantitated by measuring the loss of secondary structure as a function of added denaturant, e.g., guanidinium hydrochloride (GuHCl) (see e.g., Pace (1975) CRC Crit. Rev. Biochem. 3:1–43).